Strange resonance phenomenon?

[yesway]

Hi, sorry for the not-so-descriptive headline, here is the long specification of the problem:

In a car kit handsfree set, there is a 2" speaker and a standard microphone. Both are connected via some circuitry to a cell phone. The whole thingy is housed in a small plastic box, which also serves as a holder for the phone. Now, everything works perfectly fine as long as the parts stay unassembled, that is on the labpratory desktop. But when we put the housing and the lid togehter, the last 2mm before closing, and also when screwed together, there comes a remarkable echo, microphone picks up the speakers signal and sends it back to the other person. The phenomenon does not seem to have direct coupling to the volume, but rather to the frequency response. Still we do not get it how to get rid of this. We have done a frequency sweep, but have not found any particular frequencies that are in resonance
Any advice, anyone...?

 

[uWaveFDL]

Have you tried a damping foam behind the speaker.
Maybe a speaker cavity resonance?
Just a couple of what ifs....

 

[jasonh]

Hey yesway - I just have a couple more things you might consider: The plastic box may be causing a feedback problem by "coupling" the speaker to the mic, you could test this easily by strategically placing a c-clamp at various positions on the plastic box to see if there are any improvements and determine where the box needs to be stiffened. Along the lines of cavity resonance, you might also consider damping the mic attachment to the plastic box. Otherwise, the addition of the baffling material suggestion offered by uWaveFDL sounds promising.